1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device such as an IGBT or a thyristor.
2. Description of the Related Art
In general, a semiconductor device such as an IGBT or a thyristor is widely used as a switching element such as an inverter for controlling, for example, a motor. In order to diminish the power conversion loss, the switching element of this type is required to be operated at a high speed and to be turned on at a low forward voltage.
FIG. 1 shows an n.sup.- -channel type IGBT widely known to the art. As shown in the drawing, the conventional n.sup.- -channel type IGBT comprises a gate electrode 1, emitter electrodes 2, 3, a gate oxide film 4, emitter regions 5, 6, p-type base regions 7, 8, an n.sup.- -type layer 9, an n.sup.+ -type buffer layer 10, and a collector electrode 12. In manufacturing the IGBT shown in FIG. 1, used is a wafer comprising a p-type semiconductor layer of a high impurity concentration, an n-type semiconductor layer of a high impurity concentration formed on the p-type semiconductor layer, and an n-type semiconductor layer of a low impurity concentration formed on the n-type layer of the high impurity concentration. The IGBT is obtained by forming element regions in the surface region of the wafer by means of thermal diffusion of impurities.
FIG. 2 shows a conventional anode short-circuit type IGBT. The anode short-circuit type IGBT shown in FIG. 2 is substantially equal to the IGBT shown in FIG. 1, except that, in the IGBT shown in FIG. 2, the p.sup.+ -type layer 11 in FIG. 1 is replaced by a layer 13 consisting of p-type regions and n-type regions which are formed alternately. Incidentally, the reference numerals common with FIGS. 1 and 2 denote the same members of the IGBT. The anode short-circuit type IGBT of this type is superior to the ordinary IGBT in the relationship between the switching characteristics and the on-voltage, particularly, in the relationship on the side of a low current.
In the manufacture of an ordinary semiconductor element, it is necessary for the semiconductor wafer used to have a thickness of at least about 250 .mu.m where the wafer has a diameter of 5 inches. It should be noted that various steps such as a diffusion step, an etching step and a patterning step are involved in the manufacture of the semiconductor element. Naturally, the semiconductor wafer is transferred several times in the manufacturing process of the semiconductor element, with the result that, the wafer, if unduly thin, is likely to be cracked in the transfer step.
When it comes to the IGBT, the optimum thickness of the n.sup.+ -type buffer layer 10 shown in FIG. 2 is about 15 .mu.m. On the other hand, the optimum thickness of the n.sup.- -type layer 9 depends on the grade of the withstand voltage. In the case of an IGBT having a withstand voltage of 1200 V, which has a large demand, the optimum thickness of the n.sup.- -type layer 9 is about 100 .mu.m. In this case, the total thickness of the n.sup.- -type layer 9 and the n.sup.+ -type buffer layer 10 is only about 115 .mu.m. It follows that, in the case of manufacturing an IGBT element by using a wafer having a diameter of 5 inches, it is necessary for the layer 13 to have a thickness of at least about 140 .mu.m. What should be noted is that, if the layer 13 has such a large thickness, it is practically impossible to diffuse an n-type impurity through the back surface of the wafer to form an anode short-circuit structure as shown in FIG. 2. To be more specific, a very long diffusion time is required for the n-type impurity diffused through the back surface of the wafer to reach the n.sup.+ -type buffer layer 10. In addition, the impurity in the n.sup.+ -type buffer layer 10 is also diffused during the diffusion step, resulting in failure to obtain an ideal concentration profile of the impurity in the n.sup.+ -type buffer layer 10. Such being the situation, it was substantially impossible to obtain an anode short-circuit structure exhibiting satisfactory characteristics.